The present invention relates generally to a means for supplying engine fuel compositions and their use in spark ignition, internal combustion engines especially those having a compression ratio (CR) of 11 or more. The engine fuel supply is tailored to meet the engine""s drive cycle conditions, including load and speed.
Both petroleum refineries and engine manufacturers are constantly faced with the challenge of continually improving their products to meet increasingly severe governmental efficiency and emission requirements, and consumers"" desires for enhanced performance. For example, in producing a fuel suitable for use in an internal combustion engine, petroleum producers blend a plurality of hydrocarbon containing streams to produce a product that will meet governmental combustion emission regulations and the engine manufacturers performance fuel criteria, such as research octane number (RON). Similarly, engine manufacturers conventionally design spark ignition type internal combustion engines around the properties of the fuel. For example, engine manufacturers endeavor to inhibit to the maximum extent possible the phenomenon of auto-ignition which typically results in knocking and, potentially engine damage, when a fuel with insufficient knock-resistance is combusted in the engine.
Under typical driving situations, engines operate under a wide range of conditions depending on many factors including ambient conditions (air temperature, humidity, etc.), vehicle load, speed, rate of acceleration, and the like. Engine manufacturers and fuel blenders have to design products which perform well under virtually all such diverse conditions. This requires compromise, as often times fuel properties or engine parameters that are desirable under certain speed/load conditions prove detrimental to overall performance at other speed/load conditions. Conventionally, vehicular fuels are supplied in two or three grades, typically distinguished by their Research Octane Number, or RON. Generally, the selection of fuel grade is based upon the engine specifications. However, once the fuel is xe2x80x9con boardxe2x80x9d, it becomes a xe2x80x9cone fuel fits allxe2x80x9d and must be designed to accommodate diverse speed, load and other driving conditions.
One object of this invention is to employ a fuel supply system using a membrane for segregating octane boosting constituents from a main fuel tank or reservoir, then admixing the segregated fuel to the main fuel supply in response to engine drive cycle conditions.
Another object of this invention to establish a procedure for providing an engine with fuels specifically designed to enhance engine performance at low and high load engine conditions from a single fuel delivered to the vehicle.
Also, spark ignition engines are generally designed to operate at a compression ratio (CR) of 10:1 or lower to prevent knocking at high load. Compression Ratio (CR) is defined as the volume of the cylinder and combustion chamber when the piston is at Bottom Dead Center (BDC) divided by the volume when the piston is at Top Dead Center (TDC). As is known, higher CRs, up to about 18:1, are optimum from the standpoint of maximizing the engine thermal efficiency across the load range. A higher CR leads to greater thermal efficiency by maximizing the work obtainable from the theoretical Otto (engine compression/expansion) cycle. Higher CRs also lead to increased burn rates, giving a further improvement in thermal efficiency by creating a closer approach to this ideal Otto cycle. The use of high compression ratio spark ignition engines, however, is limited by insufficiently high fuel octane, as in practice it is difficult to supply a single fuel with sufficiently high octane overall to allow for a significant increase in compression ratio without having engine knocking at high loads.
Therefore, another objective of this invention is to facilitate the use of high compression ratio engines that realize greater thermal efficiency across the entire driving cycle without the problem of knocking at high load by supplying a specifically formulated fuel derived from the fuel supplied to the vehicle.
In theory, higher efficiency engine operation at certain moderate to high loads can be achieved by adjusting the spark ignition timing closer to the value that provides minimum spark advance for best torque or maximum brake torque (MBT). Experience has shown, however, that adjusting the ignition timing to allow MBT to be reached is not practical since knocking typically occurs under conditions of moderate to high load at timings earlier than MBT with commercially available gasolines. In principle, operating with a very high octane fuel would allow running the engine at MBT across the drive cycle. However, a preferred approach is to supply the engine with a fuel that has sufficient octane to approach or operate at MBT without knocking, together with other combustion properties tailored to optimize performance. The fuel supply system taught herein separates or extracts constituents of the supply fuel that have or can impart sufficient octane to approach or operate at MBT under varied engine load conditions.
Yet another object of the invention is to provide fuel compositions that allow adjusting the spark ignition timing closer to that which provides MBT.
Presently spark ignition engines are capable of operating with known fuels at a normalized fuel to air ratio (xe2x80x9cxcfx86xe2x80x9d) below 1.0 under low to moderate load conditions. The normalized fuel to air ratio is the actual fuel to air ratio divided by the stoichiometric fuel to air ratio. In addition, these engines can be operated with exhaust gas recycle (EGR), as the xe2x80x9cleaning outxe2x80x9d diluent, at a xcfx86 of 1.0 or lower. EGR is understood to include both recycled exhaust gases as well as residual combustion gases. An obstacle to operating the engine under such lean conditions is the difficulty of establishing a rapid and complete burn of the fuel.
Another object of this invention therefore is to provide a lower octane, lower autoignition resistant, higher laminar flame speed, high burn rate fuel for use under lean conditions to shorten the burn duration and thereby improve the thermodynamic efficiency. A faster burn rate also serves to maximize conversion of the fuel, thereby increasing the overall fuel economy and reducing emissions. As known in the art, autoignition of the fuel at sufficiently high loads can pose a threat of mechanical damage to the engine, i.e., knocking. However, at certain low load conditions, for example lean stratified operation, autoignition of the fuel can be beneficial to overall engine operation by optimizing burn characteristics that result in reduced engine emissions and higher efficiency.
The membrane separation or segregation process entails contacting a surface of the membrane with the feed material. Membrane composition is selected to permeate specific constituents of the feed. Those constituents sorbs onto and into the membrane surface region. These constituents then diffuse or migrate to the opposite surface of the membrane.
Other objects of the invention and their attendant advantages will be apparent from the reading of this specification.
One aspect of the invention is a system for separating or segregating fuel constituents from the primary fuel tank of a vehicle to supply particularly formulated fuel to meet engine needs under varying drive cycle conditions. An objective of the invention is the provision of a plurality of unleaded fuel compositions separated or segregated from a single fuel supply or reservoir, for use in operating a spark ignition, internal combustion engine, especially an engine having a CR of 11 or more, each of which compositions have different predetermined combustion properties suitable for use under preselected engine operating conditions to improve one or more of fuel efficiency and combustion emissions. The invention uses a membrane to separate an aromatics rich fuel from the fuel contained in the primary tank or reservoir. The membrane functions to preferentially separate aromatic permeate from the remaining retentate. The high aromatic content permeate provides a source of increased RON fuel that is selectively admixed or run separately to the engine fuel supply at high and moderate engine load conditions.
In one embodiment a pervaporation membrane process is employed to segregate at least a first and second fuel composition from a single fuel tank, the first fuel having combustion properties sufficient to improve combustion thereof (relative to the original tank fuel) under high and moderate engine load conditions and the second fuel having combustion properties sufficient to operate the engine under low engine load conditions.
The pervaporation process, as generally known in the art, relies upon vacuum on the permeate side of the membrane to evaporate the permeate from the surface of the membrane. The vapor phase permeate may then be condensed to liquid form.
Especially preferred fuels for use under low load conditions are those unleaded fuels boiling in the gasoline boiling range that have a RON less than 90 and an average burn rate in the engine, defined as 1/crank angles for 90% burn completion,  greater than 105% of isooctane at this time in the engine operating cycle and a laminar flame speed  greater than 105% of isooctane measured at a temperature and pressure representative of conditions in the engine at or about this time in the engine operating cycle.
Especially preferred fuels for use under high load conditions are those unleaded fuels boiling in the gasoline boiling range that have a RON greater than 100 and an average burn rate in the engine, defined as 1/crank angles for 90% burn completion,  greater than 105%% of isooctane at this time in the cycle and a laminar flame speed  greater than 105%% of isooctane measured at a temperature and pressure representative of conditions engine at or about this time in the engine operating cycle.
In view of the foregoing it will be readily appreciated that a wide range of modifications and variations of the invention are within the broad aspects set forth above and the unique scope of the invention will become even more apparent upon a reading of the detailed description which follows.